ReviewLeaching of chromated copper arsenate wood preservatives: a review
Introduction
During the development of freshwater, estuarine and marine coastlines, considerable quantities of wood are used in the construction of docks, pilings and bulkheads. Environmental pressures are increasingly inhibiting the continued use of naturally durable hardwood timbers in these structures (Eaton and Hale, 1993). However, many features of wood make it a particularly attractive building material; wood is a renewable resource, has excellent strength-to-weight properties, has a relatively low price and is easily produced (Desch and Dinwoodie, 1996). Timbers that are not naturally durable are treated with preservatives to prevent decay by wood-boring crustaceans, molluscs and fungi.
Currently, the most widely used wood preservative for timbers exposed in aquatic environments is chromated copper arsenate (CCA). CCA belongs to a group of inorganic, waterborne preservatives including chromated copper boron, ammoniacal copper arsenate, acid copper chromate, ammoniacal copper zinc arsenate and ammoniacal copper quaternary. This group has largely replaced alternative organic preservative types such as creosote, coal tars and pentachlorophenol for aquatic use, due to the environmental and human health concerns of these chemical types, as well as rising costs and declining availability of those treatments.
The metal elements in CCA are usually present in the form of oxides, and wood is industrially treated using a vacuum-pressure impregnation process according to British Standard guidelines (BSI, 1987a, BSI, 1987b, BSI, 1989). The treatment and use of preservative-treated timber is also subject to industry and international guidelines (Environment Canada, 1988, UNEP, 1994, BWPDA, WWPA, 1996). Three CCA formulations, referred to as types A, B and C, have been developed, although type C is now the most commercially popular (Table 1). Minimum lifespans in fresh and marine water are considered to be 30 and 15 years, respectively (BSI, 1989).
Out of the 591 million cubic feet (16.7 million m3) of wood preserved in the USA in 1996, 467 million cubic feet (13.2 million m3) (79.1%) were treated with waterborne preservative types. Approximately 144 million lbs (65.3 million kg) of CCA solution was used, while other waterborne preservatives amounted to 4.3 million lbs (1.9 million kg). Nearly 19 million board feet (5.8 million board m) of preserved timber were prepared for marine construction, of which 95% was treated with CCA (AWPI, 1997).
CCA-treated wood has been used extensively for over 60 years and its success as a building material suggests that leaching may not be a problem in terms of long-term efficacy. Wood preservation is an important industry in Europe and North America, with annual gross sales in the USA of around $3.91×109 (3.61×109 Euros) in 1996 (AWPI, 1997). However, recent toxicity testing studies have suggested that leaching of preservative components from wood used in aquatic situations may be harmful to the environment, particularly with the proliferation of residential docks around North American coastal waterways.
The toxicity of copper (Cu), chromium (Cr) and arsenic (As) to aquatic organisms is well recorded (Bodek et al., 1988a, Bodek et al., 1988b, Fleming & Trevors, 1989, Wong & Chang, 1991, Havens, 1994, Nriagu, 1994a, Nriagu, 1994b, Walley et al., 1996a, Walley et al., 1996b), and all are listed as priority pollutants by the United States Environmental Protection Agency (Weis et al., 1992, Weis & Weis, 1995). The reactions that take place in the wood during the fixation of CCA have a great influence on the metal species that are emitted from the wood, and the subsequent toxicity of these leachates. The toxicity of Cu, Cr and As is highly dependent on the specific form present. Cr in the +6 oxidation state is known to be carcinogenic and mutagenic, but if reduced to Cr (III), as during the CCA fixation process, it may be significantly less harmful (Sanders and Reidel, 1987). As may also be carcinogenic and mutagenic as well as teratogenic and, of the predominant oxidation states, As (V) is thought to be the more prevalent and less toxic form than As (III). Due to its chemical similarity to phosphate, arsenate may have an elevated rate of uptake by phytoplankton (Sanders and Windom, 1980), and it has been suggested that in low phosphate marine environments, arsenate may actually be more toxic than arsenite (W.S. Atkins Environment, 1998). Although Cu is an important micronutrient, it is toxic in the free ionic state above trace levels, though it may be largely partitioned to organic material in the aquatic environment, particularly humic acids (Newell & Sanders, 1986, Fleming & Trevors, 1989, Livens, 1991, Hung et al., 1993).
Studies have been conducted exposing marine organisms to CCA-treated wood or leachate waters and deleterious effects have been shown against a range of aquatic organisms (Weis et al., 1991, Weis et al., 1992. Criticism of this work focused on the unrealistically high ratio between wood and water volume, which allowed the metal concentrations to build up to toxic levels (Albuquerque & Cragg, 1995a, Albuquerque & Cragg, 1995b, Breslin & Adler-Ivanbrook, 1998). Further work has suggested a decrease in biodiversity close to CCA-treated marine structures, and elevated levels of metal elements in benthic organisms (Weis & Weis, 1994a, Weis & Weis, 1994b, Weis & Weis, 1995, Weis & Weis, 1996, Albuquerque & Cragg, 1995a, Albuquerque & Cragg, 1995b, Wendt et al., 1996, Cragg & Eaton, 1997, Weis et al., 1998). Although Cu concentrations were found to be significantly elevated in algae growing on CCA-treated wood panels, no increase was found in fish species associated with the same panels (Weis and Weis, 1999). This suggests that trophic transfer to consumers did not occur, although it was possible that the duration of the studies was insufficient to allow accumulation in higher consumers. Similarly, Adler-Ivanbrook and Breslin (1999) found little metal accumulation in blue mussels exposed to treated wood panels in laboratory and field exposures. Again, experimental design may have influenced results, where continuous flushing of the laboratory system may have prevented bioaccumulation.
In contrast, leachates from untreated wood were shown to have a greater toxicity towards fish and invertebrates than leachates from CCA-treated wood. The adverse effects noted were thought to be due to naturally occurring extractives including aldehydes, phenols, terpinene, camphene and pinene (Baldwin et al., 1996, Taylor et al., 1996). These naturally occurring extractives may be leached out somewhat during the treatment process, or may be more strongly bound to wood as a result of complex formation during treatment.
One of the major problems is that due to inadequate understanding of long-term leaching rates, recommended preservative loading is presently set at very high levels. For example, common treated timbers such as Scots pine and Douglas fir have densities between 500 and 550 kg m−3 (Desch and Dinwoodie, 1996). Therefore, with a salt loading of up to 50 kg m−3 recommended (BSI, 1989, Eaton & Hale, 1993), the preservative may represent around 10% of the final timber weight. It is unclear from the current literature if these levels are based on toxicity thresholds of common decay organisms, or are merely intended to account for losses expected throughout the service period. Preservatives must be persistent enough to allow protection throughout the predicted lifespan of the structure, which may be up to 30 years in fresh water conditions. The active components must be of low solubility to resist leaching, yet soluble enough to continue to be effective against organisms responsible for decay (Hegarty and Curran, 1986).
In addition to the possible environmental problems of losses of preservative components during the life of timber, disposal of timbers still retaining high levels of preservative is also of concern. In Germany and France, around 2.1–2.4 million tons of wood waste is considered dangerous (according to the European Council directive 91/689/EEC on hazardous waste). In France alone, out of 25 million CCA-treated poles, 500,000 (or 50,000 tons) are removed from service annually and must be disposed of (Helsen and Van den Bulck, 1998). Better understanding of losses in service may facilitate a reduction in initial loading, and thus alleviate the problems of disposal.
The wood preserving industry is also engaged in a considerable research and development programme to generate improved biocides. A number of these are based on Cu, with the Cr and As replaced by a triazole biocide in copper azole, or a quaternary ammonium biocide in ammoniacal copper quaternary. The novel biocides, in addition to containing Cu, will bind to wood along the same ion exchange mechanisms, so better understanding of the factors that affect CCA leaching will be of benefit in their further development.
Wood preservatives are also subject to increasingly stringent environmental legislation, particularly within the European Union where they will fall under the control of the new Biocidal Products Directive, which will come into force in the year 2000. As-containing wood preservatives have also recently been scrutinised under the Marketing and Use Directive (W.S. Atkins Environment, 1998).
Wood preservatives must be considered as part of a much wider suite of biocides. Of these, the adverse environmental effects of antifouling paint biocides containing organotins have been widely reported (Gibbs et al., 1987, Gibbs et al., 1988, Clark et al., 1988). Cu remains an important active ingredient in the antifouling paint industry following legislation against the use of organotin compounds, and is also likely to remain a mainstay of the wood preserving industry in the future. Closer examination of the wood preservatives as an additional source of Cu to the aquatic environment is therefore relevant.
To enable a more realistic assessment of the possible environmental effects of CCA-treated timber accurate quantification of component leaching rates is required. Leaching involves a number of different processes, including initial loss of surface deposits and unfixed components, penetration of water into wood and hydrolysis or dissolution of the fixed or complexed components and migration of preservative to the surface of the wood (Cooper, 1994). Aspects of the preservative treatment of wood may affect its leachability, in addition to the environment the wood is exposed to in its period of service.
A comprehensive literature review conducted for the United States Department of Agriculture provides a summary of pertinent data available up to 1995 on leaching of a number of preservative types in terrestrial and aquatic environments (Lebow, 1996). This earlier review concluded that despite the numerous laboratory studies that had been conducted, the data generated often had little applicability to in-service leaching rates. It also highlighted the need for further research to address the effects of different environmental exposures, such as fresh water, seawater and highly organic environments, and the need to monitor the overall environmental fate of leached wood preservative components.
The aims of this review are to evaluate the existing data on leaching of CCA, and the principal factors that affect leaching rates in order that releases to different aquatic environments can be predicted and risks assessed. While much of the literature reviewed concerns research conducted with CCA applications in terrestrial as well as aquatic environments, the authors have made every effort to rely upon work which has a specific aquatic focus.
Section snippets
Fixation
Although the fixation of CCA is still not completely understood, the process is generally defined by the reduction of hexavalent chromium. The reduction of the reactive and mobile Cr (VI) to Cr (III) is crucial in the formation of insoluble complexes in CCA-treated wood. As can be seen in Table 2, there is a direct correlation between the level of unreacted Cr (VI) in treated wood and the leaching concentration of CCA components and complete fixation is essential to minimise leaching (Cooper et
Leaching
There has been a considerable amount of literature published concerning leaching of CCA wood preservatives in aquatic environments. However, the focus of much of the early work has been on monitoring leaching in terms of the durability of wood and the ability of treated timbers to withstand biological decay, rather than quantify releases to, and effects on, the marine environment (Fahlstrom et al., 1967, Hager, 1969, Cherian et al., 1979, Johnson, 1982, Eaton, 1989, Green et al.,1989). Although
Component redistribution
Work has also been done to try to quantify movement of individual components within the wood during leaching trials. Cu concentrations were observed to increase significantly in the peripheral zones, with large-scale depletion from the inner sections, in long-term trials with marine piles in New Zealand (McQuire, 1976). Similarly, although loss of Cu was greater from the outer zone after just 6 months marine exposure, between 12 and 85 months exposure losses were much greater from the inner
Speciation
Although considerable information exists on speciation of the individual elements in CCA, there is little research specific to CCA leachates (Albuquerque & Cragg, 1995a, Albuquerque & Cragg, 1995b). It is not clear if metals are leached as individual elements, as Cu or Cr arsenates, as inorganic complexes or possibly even as organometallic complexes bound to water-soluble wood extractives (Lebow, 1996).
Baldwin et al. (1996) studied partitioning of metals to sediment during laboratory leaching
Leaching mechanism
Just as a general scheme for the fixation of CCA has been developed (Table 3) it is possible to propose a similar scheme to describe the possible leaching mechanism, based on the published literature (Table 6). The principal factors likely to affect each reaction stage are also included.
The mid-term solvation of crytallite forms of Cu may explain the relatively high leaching rate for this element compared with the other metals. The longer-term reactions and redistribution of elements may take
Conclusions
Aspects of both the preservative treatment of wood and the environmental conditions the wood is exposed to may affect its leachability. Factors such as preservative formulation, fixation temperature, post-treatment handling, timber dimensions and leaching media pH, salinity and temperature have been shown to affect leaching rates. However, more rigorous examination of these factors is required if accurate prediction of in-service leaching rates is to be made based solely on results of
Acknowledgements
One of the authors (J.A.H.) is grateful to the Natural Environment Research Council for partial funding of a CASE studentship in association with Laporte Industries.
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